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ATM S 111, Global Warming:
Understanding the Forecast
DARGAN M. W. FRIERSON
DEPARTMENT OF ATMOSPHERIC SCIENCES
DAY 3: 04/06/2010
Climate Forcings vs Climate Feedbacks
 Climate forcings:
 Things that change global temperatures directly
 Climate feedbacks:
 Things that respond to temperature changes (but themselves
affect temperature too!)
Outline of This Topic
 Climate forcings
 Things that directly change global temperature
 How to put different effects on the same ground


Radiative forcing will be a key concept
Forcings important for climate

Including greenhouse gases, land cover changes, volcanoes, air
pollution, and others…
 It’s a long list!
 Notion of “global warming” versus “climate change” will become
more and more apparent
Outline of This Topic
 Climate feedbacks
 Things that change in response to a climate forcing
Ice/snow
 Ice melts when it gets hotter: classic example of feedback
 Water vapor
 Clouds


Why is there uncertainty in climate forecasts?
Climate Forcings
 Climate forcings directly change global temperatures
 Examples:
 Changes in strength of the Sun
 Changes in greenhouse gas concentrations
 Volcanoes (which block out the Sun)
 We need a way to put these on equal footing in terms
of how much warming they cause

Let’s recall how the Earth is heated/cooled
Heating/Cooling of the Earth
 Solar radiation in, longwave radiation out
 The Earth is heated by shortwave radiation from the Sun
 The Earth cools by longwave radiation
Radiative Forcing
 Radiative forcing is calculated as the change in
shortwave in or longwave out due to the
particular climate forcing




Measured in Watts per meter squared (W/m2)
An increase in shortwave radiation absorbed by Earth is a
positive radiative forcing
A decrease in longwave out of Earth is a positive radiative
forcing
In response to a positive radiative forcing, the climate must
warm
Radiative Forcings: Shortwave Forcings
 Radiative forcing: change in shortwave in or
longwave out due to the particular climate forcing

For shortwave forcings, this is just the change in solar
energy absorbed by the planet
Ex. 1: if the Sun increases in strength so 0.2 W/m2 more is
absorbed, the radiative forcing is 0.2 W/m2
 OK that was obvious…
 Ex. 2: if a volcano blows up and reflects back an extra 0.3 W/m2 of
the Sun’s rays, the radiative forcing is -0.3 W/m2

Radiative Forcing: Longwave Forcings
 OK, so shortwave forcings were pretty easy
 What about gases that affect the greenhouse effect?
 Radiative forcing for greenhouse gases:
 Instantly change the gas concentration as compared with a
reference concentration (typically “preindustrial” values from the
year 1750)


E.g., compare current CO2 levels with preindustrial times
Calculate how much longwave radiation to space is decreased
Have to assume temperature is unchanged too
 Ex: When increasing the concentration of a certain greenhouse gas,
longwave radiation is decreased by 2 W/m2 due to this gas

Radiative Forcings
 In response to a positive radiative forcing, the system
will heat up


And therefore will radiate more to space
Thus radiative forcing for greenhouse gases is calculated
assuming no change in temperature
 Ex: CO2 levels are increased to decrease the
longwave radiation to space by 4 W/m2


The atmosphere will heat up in response (because shortwave is
greater than longwave)
It will radiate away more, eventually getting into energy
balance
Climate Forcing of CO2
 Radiative forcing of CO2 for current value versus
preindustrial (year 1750) value: 1.66 W/m2
 Radiative forcing for doubling CO2: around 3.7 W/m2

And the radiative forcing increase gets less as CO2 increases more
Radiative Forcing of Other Greenhouse Gases
 These are all current values vs preindustrial values
Carbon dioxide:
Methane:
Nitrous oxide:
CFCs:

1.66 W/m2
0.48 W/m2
0.16 W/m2
0.32 W/m2
But CFCs are decreasing now (everything else is increasing)
 These numbers are how I calculated the percentages
of the anthropogenic greenhouse effect from last
lecture…
Climate Forcings: Shortwave Forcings
 Those were the major longwave radiative forcings
 Let’s do shortwave forcings next
 Shortwave forcings affect how much solar radiation is
absorbed)
 Examples of shortwave forcings:
 Changes in strength of the Sun
 Changes in the surface albedo



Not changes in ice coverage – that’s a feedback
Volcanoes
Air pollution

This falls under the more general category of “aerosols”
 Let’s discuss each of these in more detail
Changes in the Intensity of the Sun
 The Sun has natural variability in its strength
 The changes are rather small though (around 0.1%) since we’ve
been measuring accurately with satellites (1979)
 The variability of the Sun is
correlated with the
sunspot cycle

Sunspots are temporarily
darkened regions on the Sun 
Sunspot Cycle
 Sunspots vary over an 11 year cycle
 More sunspots  more solar radiation
 FF: Also more solar flares during solar cycles

These mess with satellites, communication systems, etc
Solar
output
FF: Radiative forcing by the Sun
= change in solar radiation
absorbed on Earth
= 0.7*(irradiance change)/4
= 0.2 W/m2 (max to min)
Sunspot
number
(takes into account albedo and
directness of radiation)
1975
Year
Current Solar Intensity
 We’re at the end of a deep minimum of solar intensity
Peak to peak radiative
forcing of the Sun:
0.2 W/m2
Affects global
temperatures by around
0.2o C (research of Prof.
Tung, Applied Math, UW)
Sunspot Cycles over Time
 Sunspot cycles are not the same each 11 year cycle:
 Global temperature
 Carbon dioxide
 Sunspots
Solar variability may have led to some
warming in the early 20th century
Sunspots over Last 400 Years
 Sunspots since we’ve been observing them:
A Preview of Some Paleoclimate
 Maunder minimum (1650-1700) coincides with
“Little Ice Age”:
There was also
enhanced volcanic
activity at this time
We’ll discuss these & other
periods more when
we study paleoclimate
Other Ways to Change Absorbed Solar Radiation
 Changes in the Sun aren’t the only way to change
absorbed solar radiation
 We can also directly change the albedo of the Earth



Land cover
Soot on snow
Reflective particles in the air
Land Cover Changes
 Forests have low albedo (they’re dark)
 Cutting down forests to create farmland/pastures
tends to raise the albedo

This is actually a negative
radiative forcing
Causes local cooling because
there’s more solar energy reflected


Remember that deforestation
is an important source of
carbon dioxide though

Deforestation can cause global
warming but local cooling…
Princeton, NJ
Soot on Snow
 A tiny amount of soot (AKA black carbon) in pure
white snow can change the albedo dramatically!

Currently a very active area of research (Prof. Warren, Atmos Sci)
Fresh snow over Greenland
from high above
Other Ways to Change Albedo
 Can change albedo in the atmosphere as well!
 Aerosols (fine particles suspended in air) make a
large contribution to reflection of sunlight




Volcanoes!
Pollution (from coal burning or other types of burning)
Dust (e.g., from the Sahara)
And others
Volcano Effects on Climate
 Volcanoes can have a large climate impact
 Certain big ones cause a temporary cooling of the climate
Mount Pinatubo, Philippines,
erupted June 1991, resulted in
more than 0.5o C (0.9o F) global
temperature decrease
Direct heating of atmosphere by
volcanoes is small.
CO2 emission by volcanoes is
<1% of anthropogenic emission.
Volcano Impacts on Climate
 Dust and sulfates from volcanoes block out the Sun
Before the
Pinatubo
eruption 
2 months
after 
Volcanic material spreads quickly
around the same latitudes as the
eruption
Slight dimming seen across the
globe over a year after the eruption
14 months
after 
Red colors = atmosphere is
reflecting a lot of sunlight back
Volcano Impacts on Climate
 Effects of big eruptions are felt for a couple of years
Temperatures were cold for around 2
years before recovering
Fun Fact: Tropical volcanoes that get
lots of sulfates into the stratosphere
have the biggest climate effect
Observed (dashed) vs
modeled (solid) temperature
change (from Hansen et al 1996)
Air Pollution Aerosols
 Air pollution particles block out sunlight too
 Sulfates from dirty coal burning are particularly important
(sulfate aerosols)


This is the same stuff that causes acid rain
These are a big effect

One of the main uncertainties in our understanding of climate
Aerosol “direct effect”
Aerosol haze (here from
east coast) reflect sunlight
back to space and cool.
They fall out over a few
days.
Aerosol “Indirect Effect”
 Aerosols also affect
cloud formation
 Ship tracks can be
seen as brighter
clouds follow the
ships’ smokestacks
“Global Dimming”
 Solar radiation reaching the Earth’s surface
declined by ~4% from 1961-1990

This has coexisted with large increases in the global
temperature. Why?
 Increased aerosol concentrations partially to blame
 Both direct reflection and indirect cloud changes are thought
to be important
 Trend has reduced since 1990s (likely due to Clean
Air Act and similar laws in Europe)
Aerosol Effects on Climate
 Air pollution thus is another strange issue:
 Sulfate aerosols reflect away sunlight so itself causes cooling
 Cleaning up pollution has had great benefits for air quality,
human health, acid rain, etc
 However it has likely led to additional warming
Aerosol increases/cleanup is likely
partially to blame for the small
warming from 1950-1970, and the
rapid warming since then
Another twist: China is pumping
out lots of dirty coal emissions now
Summary of Shortwave Climate Forcings
 Shortwave radiative forcings can come from:
 Changes in strength of the Sun
 Changes in albedo at the surface
 Changes in albedo of the atmosphere
Summary of Shortwave Climate Forcings
 Radiative forcings for shortwave agents in current
climate vs preindustrial






Remember CO2 radiative forcing is currently:
1.66 W/m2
Solar radiation changes:
0.12 W/m2
Land cover changes:
-0.20 W/m2
Soot on snow:
0.10 W/m2
Aerosol direct effect:
-0.50 W/m2
Aerosol indirect effect (clouds):
-0.70
W/m2
 All of the above have significant scientific
uncertainty associated with them though!

We just don’t know these values very accurately
Radiative Forcings all Plotted Together
 Red = warming, blue = cooling
 Longwave and shortwave together here
Radiative forcing of
present climate vs
preindustrial, with
Uncertainties
Aerosols (air pollution)
are the biggest
uncertainty
IPCC AR4 SPM
Climate Forcings Summary
 Climate forcings either change shortwave radiation
or longwave radiation

Longwave forcings are greenhouse gases and include:
Carbon dioxide
 Methane
 Nitrous oxide
 Ozone


Shortwave forcings include:
Changes in solar radiation
 Changes in surface albedo by land use and soot on snow
 Volcanoes
 Aerosols

Local Aspects of Many Climate Forcings
 CO2 is still the main problem
 And it is global (essentially the same concentration
everywhere)
 Hence “global warming” is an appropriate name
 Many of the other climate forcings are much more
localized though


Soot on snow, land use, aerosols all tend to be localized
Hence “climate change” is a better term when covering these
Next: Climate Feedbacks
 Things that change when the climate gets warmer or
colder

We’ll discuss the following:
Water vapor feedback
 Ice-albedo feedback
 Cloud feedbacks

 Feedbacks are of critical importance in determining
temperature response to climate forcings

Positive feedbacks are things that amplify warming
Climate Sensitivity
 Global warming theory:
= common symbol indicating the change in a quantity
= change in temperature (in degrees C)
= radiative forcing (in W/m2)
= climate sensitivity
Climate Sensitivity
 Lots of positive feedbacks means a very sensitive
climate (large

)
Large change in temperature for even a small forcing
 Lots of negative feedbacks means small
 What are the main climate feedbacks?
 And are they positive or negative?
Water Vapor Feedback
 Water vapor feedback
 Remember water vapor is the number one greenhouse gas
 This feedback is very confidently expected to be positive
 Why? Because warmer air can hold more
moisture
Surface Water Vapor Content
 Surface water vapor content
Source: NCEP Reanalysis
Most water vapor is in the tropics where it’s hot
Water Vapor Content
 Winters are much drier than summers
 Simply because cold temperatures means small water vapor
content
January surface water vapor content
July surface water vapor content
Water Vapor Feedback
 Basic idea:
 A warmer climate means a higher water vapor climate

20% more humid climate with 3o C temperature increase
 As with all feedbacks, water vapor doesn’t care what
the forcing is that caused the warming

Any kind of warming will result in an increase in water vapor
content
Water Vapor Feedback
 A warmer climate means a higher water vapor
climate
 Essentially no arguments out there that water vapor
should act as a negative feedback

Some reasonable skeptics argue that the feedback might be
weak


Arguments focus on how upper atmospheric water vapor might
change
Observations show evidence for a strong positive feedback

Water vapor increases/decreases right along with global
temperatures
Ice/Snow Feedbacks
 Can you think of a feedback involving ice/snow?
Arctic sea ice
Ice-Albedo Feedback
 Warming  ice melting  dark open ocean visible
 more warming
 Similar feedback is
present for snow
(revealing darker
land surfaces below)
Cloud Feedbacks
 Cloud feedbacks are much more uncertain
 Partially because clouds have both an albedo effect and a
greenhouse effect
High (thin) Clouds Warm
Low (thick) Clouds Cool
Cloud Feedbacks
 Cloud feedbacks lead to the largest uncertainty in
global warming forecasts


More low clouds could lead to less warming than predicted
However, roughly equally likely, less low clouds could lead to
significantly more warming…
 Uncertainty: a reason not to act or to act quickly?
Lapse Rate Feedback
 Main negative feedback: “lapse rate feedback”
Height (km)
Upper atmosphere warms faster
than lower atmosphere in climate
models
This is where longwave radiation to
space comes from
Funny skeptic argument: upper
atmosphere wasn’t warming as fast as
Expected (resolved by Professor Qiang
Fu and Celeste Johanson at UW):
If it had been true, this would have
meant we’d expect more warming!
Feedbacks and Climate Sensitivity
 Climate models say that expected warming is
approximately double that expected with no
feedbacks


Warming response to doubling CO2 (we’ll likely get to this
around 2050) with no feedbacks is around 1.5o C
Models predict 3o C average response to warming
 There’s some uncertainty in the feedbacks though
 And it’s hard to rule out high sensitivity climates
Uncertainty in Feedbacks
 Since positive feedbacks combine, high sensitivity
climates are hard to rule out (work of Prof. Roe, ESS)
From 6,000 simulations
randomly changing model
parameters
Likelihood
Very high temperature
changes (e.g., 8o C) are
unlikely, but hard to rule out
(on the other hand, small
temperature changes like
1o C are essentially impossible)
Summary
 Radiative forcing: key method to size up
shortwave and longwave climate forcings
 Longwave forcings: greenhouse gases
 Shortwave forcings:




Solar variations
Land use changes
Soot on snow
Aerosols: key uncertainty
Summary
 Feedbacks:
 Water vapor feedback is positive
 Ice-albedo feedback is positive
 Cloud feedback is another key uncertainty
 High sensitivity climates are hard to rule out